1. Field of the Invention
The present invention relates to a coating agent for a film, a process for coating the coating agent on film, and the laminate film produced thereby. The laminate film can be used as a wrapping film, for example, for wrapping food.
2. Description of the Related Art
Various types of oxygen gas barrier films, and wrapping materials containing oxygen gas barrier films, are known. Aluminum (Al) foil has perfect oxygen gas barrier properties, but when used by itself, it often has pin holes. Therefore, Al foil is used as the interlayer of laminate films. However, since laminate films containing an Al foil layer are not transparent, it is difficult to see the contents of packages made with such films. Furthermore, packages made from films containing an Al foil layer cannot be used in applications in which it is desirable to use metal detectors to identify metal contamination of the contents. Finally, films containing an Al foil layer generate Al residues after incineration.
Other known oxygen gas barrier films are polyvinylidene chloride (PVDC) films and PVDC-coated films. PVDC absorbs little moisture and exhibits good gas barrier properties even in high-humidity conditions, and is therefore used for coating various substrates. The substrates to be coated with PVDC include films of, for example, biaxially-oriented polypropylene (OPP), biaxially-oriented nylon (ON), biaxially-oriented polyethylene terephthalate (OPET), and cellophane, etc. However, PVDC-laminated films are problematic in that they generate hydrogen chloride gas when incinerated as waste.
Completely hydrolyzed, non-modified polyvinyl alcohol (PVA) films are also known as oxygen gas barrier films. PVA films exhibit excellent oxygen gas barrier properties under low-humidity conditions. In general, however, they absorb moisture, and their oxygen gas barrier properties are greatly degraded in high-humidity conditions in which the relative humidity is around 70% or higher. Another problem with PVA films is that their adhesion to OPP or OPET substrate films is poor. Some proposals have been made heretofore for attenuating the moisture absorption of PVA. For example, an ethylene-vinyl alcohol copolymer (EVOH) having an ethylene comonomer content of at least 20 mol % can be used, or both surfaces of PVA films can be coated with PDVC. However, EVOH coating solutions must contain organic solvents, and the organic solvents used are detrimental to the working environment. Alternatively, PVDC-coated films are also problematic in that they generate hydrogen chloride gas when incinerated.
The following techniques are known, relating to the present invention.
(1) JP-A 4-331246 discloses gas barrier materials prepared by coating moisture-proof plastics (e.g., polyethylene, polypropylene) with a gas barrier substance capable of dissolving in water or solvents (e.g., polyvinyl alcohols having a degree of hydrolysis of at least 95 mol %, ethylene-vinyl alcohol copolymer having an ethylene content of from 28 to 51 mol %).
(2) JP-A 6-32924 discloses oxygen barrier films prepared by coating biaxially-oriented polypropylene films or biaxially-oriented polyester films with an aqueous solution of a polyvinyl alcohol having a degree of polymerization of from 300 to 1000 and a degree of hydrolysis of from 95 to 99.5 mol %, thereby forming thereon a coating layer having a thickness of from 1.7 to 4 .mu.m.
(3) U.S. Pat. No. 4,927,689 discloses gas barrier structures prepared by coating a thermoplastic resin substrate with a first coating layer of a urethane primer of from 0.3 to 3.0 g/m.sup.2 and a second coating layer of a polyvinyl alcohol of at most about 2.0 g/m.sup.2.
(4) The present applicant's own JP-A 8-188624 discloses self-sustaining films (preferred thickness: from 10 to 100 .mu.m) of a modified polyvinyl alcohol that contains from 1 to 20 mol % of .alpha.-olefin units with up to 4 carbon atoms and has a degree of polymerization of at least 700.
(5) The present applicant's own JP-A 9-241999 discloses a coating agent for paper that comprises a modified polyvinyl alcohol containing from 3 to 15 mol % of ethylene units.